They were average, to be honest. Not too bad, but better than some, that's true.
However, for example, and no offense meant, the voice actor of...
... voice actor of ghost Craig Un'shallach was terrible. :P Caine's voice acting was perhaps ok, and maybe Flink McWinter's, but others weren't that good. Maybe companion's.
But like it was said before, the voice acting in cinematics was great.
As with previous games? I loved a few voice actors in Order of Dawn. For example, when Rohen awakens you, voice acting then was pretty cool. And of course, in all of the cinematics back then, too.
<--- I loved it. But I also loved this kind of voice acting: http://www.youtube.com/watch?v=BCr7y4SLhck
(Note: I don't like WoW despite liking Warcraft, although I love their cinematics too.)